Scan order of secondary transform coefficients

ABSTRACT

Methods, apparatus, and computer readable storage medium for processing video data. The method includes extracting a data block from the video data; scanning a first number of data items in the data block following a first scan order to generate a first data sequence; performing a non-separable transform to the first data sequence to obtain a second data sequence having a second number of data items; and replacing at least a portion of the first number of data items in the data block with a portion or all of the second data sequence following a second scan order.

INCORPORATION BY REFERENCE

This application is based on and claims the benefit of priority to U.S.Provisional Application No. 63/238,646, entitled “MODIFIED SCAN ORDER OFSECONDARY TRANSFORM COEFFICIENTS”, filed on Aug. 30, 2021, which isherein incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure describes a set of advanced video coding technologies.More specifically, the disclosed technology involves implementation ofnon-separable transform of data blocks in video encoding and decoding.

BACKGROUND

This background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing of thisapplication, are neither expressly nor impliedly admitted as prior artagainst the present disclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, with each picture having a spatialdimension of, for example, 1920×1080 luminance samples and associatedfull or subsampled chrominance samples. The series of pictures can havea fixed or variable picture rate (alternatively referred to as framerate) of, for example, 60 pictures per second or 60 frames per second.Uncompressed video has specific bitrate requirements for streaming ordata processing. For example, video with a pixel resolution of1920×1080, a frame rate of 60 frames/second, and a chroma subsampling of4:2:0 at 8 bit per pixel per color channel requires close to 1.5 Gbit/sbandwidth. An hour of such video requires more than 600 GBytes ofstorage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the uncompressed input video signal, through compression.Compression can help reduce the aforementioned bandwidth and/or storagespace requirements, in some cases, by two orders of magnitude or more.Both lossless compression and lossy compression, as well as acombination thereof can be employed. Lossless compression refers totechniques where an exact copy of the original signal can bereconstructed from the compressed original signal via a decodingprocess. Lossy compression refers to coding/decoding process whereoriginal video information is not fully retained during coding and notfully recoverable during decoding. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is made smallenough to render the reconstructed signal useful for the intendedapplication albeit some information loss. In the case of video, lossycompression is widely employed in many applications. The amount oftolerable distortion depends on the application. For example, users ofcertain consumer video streaming applications may tolerate higherdistortion than users of cinematic or television broadcastingapplications. The compression ratio achievable by a particular codingalgorithm can be selected or adjusted to reflect various distortiontolerance: higher tolerable distortion generally allows for codingalgorithms that yield higher losses and higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories and steps, including, for example, motion compensation,Fourier transform, quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, a picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be referred to as an intra picture. Intra pictures and theirderivatives such as independent decoder refresh pictures, can be used toreset the decoder state and can, therefore, be used as the first picturein a coded video bitstream and a video session, or as a still image. Thesamples of a block after intra prediction can then be subject to atransform into frequency domain, and the transform coefficients sogenerated can be quantized before entropy coding. Intra predictionrepresents a technique that minimizes sample values in the pre-transformdomain. In some cases, the smaller the DC value after a transform is,and the smaller the AC coefficients are, the fewer the bits that arerequired at a given quantization step size to represent the block afterentropy coding.

Traditional intra coding such as that known from, for example, MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt coding/decoding of blocks based on, for example, surroundingsample data and/or metadata that are obtained during the encoding and/ordecoding of spatially neighboring, and that precede in decoding orderthe blocks of data being intra coded or decoded. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction uses reference data only from the currentpicture under reconstruction and not from other reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques are available in a given video coding technology,the technique in use can be referred to as an intra prediction mode. Oneor more intra prediction modes may be provided in a particular codec. Incertain cases, modes can have submodes and/or may be associated withvarious parameters, and mode/submode information and intra codingparameters for blocks of video can be coded individually or collectivelyincluded in mode codewords. Which codeword to use for a given mode,submode, and/or parameter combination can have an impact in the codingefficiency gain through intra prediction, and so can the entropy codingtechnology used to translate the codewords into a bitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). Generally, for intra prediction, a predictor block can be formedusing neighboring sample values that have become available. For example,available values of particular set of neighboring samples along certaindirection and/or lines may be copied into the predictor block. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions specified in H.265's 33 possible intra predictordirections (corresponding to the 33 angular modes of the 35 intra modesspecified in H.265). The point where the arrows converge (101)represents the sample being predicted. The arrows represent thedirection from which neighboring samples are used to predict the sampleat 101. For example, arrow (102) indicates that sample (101) ispredicted from a neighboring sample or samples to the upper right, at a45 degree angle from the horizontal direction. Similarly, arrow (103)indicates that sample (101) is predicted from a neighboring sample orsamples to the lower left of sample (101), in a 22.5 degree angle fromthe horizontal direction.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are example referencesamples that follow a similar numbering scheme. A reference sample islabelled with an R, its Y position (e.g., row index) and X position(column index) relative to block (104). In both H.264 and H.265,prediction samples adjacently neighboring the block under reconstructionare used.

Intra picture prediction of block 104 may begin by copying referencesample values from the neighboring samples according to a signaledprediction direction. For example, assuming that the coded videobitstream includes signaling that, for this block 104, indicates aprediction direction of arrow (102)—that is, samples are predicted froma prediction sample or samples to the upper right, at a 45-degree anglefrom the horizontal direction. In such a case, samples S41, S32, S23,and S14 are predicted from the same reference sample R05. Sample S44 isthen predicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has continued to develop. In H.264 (year 2003), for example,nine different direction are available for intra prediction. Thatincreased to 33 in H.265 (year 2013), and JEM/VVC/BMS, at the time ofthis disclosure, can support up to 65 directions. Experimental studieshave been conducted to help identify the most suitable intra predictiondirections, and certain techniques in the entropy coding may be used toencode those most suitable directions in a small number of bits,accepting a certain bit penalty for directions. Further, the directionsthemselves can sometimes be predicted from neighboring directions usedin the intra prediction of the neighboring blocks that have beendecoded.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions in various encoding technologies developed overtime.

The manner for mapping of bits representing intra prediction directionsto the prediction directions in the coded video bitstream may vary fromvideo coding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions for intro prediction that arestatistically less likely to occur in video content than certain otherdirections. As the goal of video compression is the reduction ofredundancy, those less likely directions will, in a well-designed videocoding technology, may be represented by a larger number of bits thanmore likely directions.

Inter picture prediction, or inter prediction may be based on motioncompensation. In motion compensation, sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), may be used for a prediction of a newly reconstructedpicture or picture part (e.g., a block). In some cases, the referencepicture can be the same as the picture currently under reconstruction.MVs may have two dimensions X and Y, or three dimensions, with the thirddimension being an indication of the reference picture in use (akin to atime dimension).

In some video compression techniques, a current MV applicable to acertain area of sample data can be predicted from other MVs, for examplefrom those other MVs that are related to other areas of the sample datathat are spatially adjacent to the area under reconstruction and precedethe current MV in decoding order. Doing so can substantially reduce theoverall amount of data required for coding the MVs by relying onremoving redundancy in correlated MVs, thereby increasing compressionefficiency. MV prediction can work effectively, for example, becausewhen coding an input video signal derived from a camera (known asnatural video) there is a statistical likelihood that areas larger thanthe area to which a single MV is applicable move in a similar directionin the video sequence and, therefore, can in some cases be predictedusing a similar motion vector derived from MVs of neighboring area. Thatresults in the actual MV for a given area to be similar or identical tothe MV predicted from the surrounding MVs. Such an MV in turn may berepresented, after entropy coding, in a smaller number of bits than whatwould be used if the MV is coded directly rather than predicted from theneighboring MV(s). In some cases, MV prediction can be an example oflossless compression of a signal (namely: the MVs) derived from theoriginal signal (namely: the sample stream). In other cases, MVprediction itself can be lossy, for example because of rounding errorswhen calculating a predictor from several surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 specifies, described below is atechnique henceforth referred to as “spatial merge”.

Specifically, referring to FIG. 2 , a current block (201) comprisessamples that have been found by the encoder during the motion searchprocess to be predictable from a previous block of the same size thathas been spatially shifted. Instead of coding that MV directly, the MVcan be derived from metadata associated with one or more referencepictures, for example from the most recent (in decoding order) referencepicture, using the MV associated with either one of five surroundingsamples, denoted A0, A1, and B0, B1, B2 (202 through 206, respectively).In H.265, the MV prediction can use predictors from the same referencepicture that the neighboring block uses.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding and decoding.

Aspects of the disclosure also provide non-transitory computer-readablemediums storing instructions which when executed by a computer for videodecoding and/or encoding cause the computer to perform the methods forvideo decoding and/or encoding.

According to one aspect, an embodiment of the present disclosureprovides a method for processing video data. The method includesextracting a data block from the video data; scanning a first number ofdata items in the data block following a first scan order to generate afirst data sequence; performing a non-separable transform to the firstdata sequence to obtain a second data sequence having a second number ofdata items; and replacing at least a portion of the first number of dataitems in the data block with a portion or all of the second datasequence following a second scan order.

According to another aspect, an embodiment of the present disclosureprovides a method for entropy encoding transform coefficients associatedwith video data. The method includes in response to a transformassociated with the transform coefficients being non-separable, scanningthe transform coefficient when performing entropy encoding of thetransform coefficients using a first scanning order being one of: ahorizontal scan order; or a vertical scan order; and in response to thetransform associated with the transform coefficients being separable,scanning the transform coefficient in a second scanning order differentfrom the first scanning order when performing entropy encoding of thetransform coefficients.

According to another aspect, an embodiment of the present disclosureprovides a method for processing video data. The method includesreceiving the video data; determining whether a non-separable transformis applied as a secondary transform to the video data; in response tothe non-separable transform being applied as the secondary transform tothe video data: scanning a first number of primary transformcoefficients, wherein the primary transform coefficients follow a firstscanning order; performing a non-separable transform using the firstnumber of primary transform coefficients as an input to obtain a secondnumber of secondary transform coefficients as an output, wherein thesecondary transform coefficients follow a second scanning order;replacing at least the second number of primary transform coefficientswith the secondary transform coefficients following the second scanningorder; performing an inverse secondary transform corresponding to thenon-separable transform using the second number of secondary transformcoefficients as an input to obtain the first number of primary transformcoefficients as an output; and replacing at least the first number ofsecondary transform coefficients with the primary transform coefficientsfollowing the first scanning order.

According to another aspect, an embodiment of the present disclosureprovides an apparatus for video encoding and/or decoding. The apparatusincludes a memory storing instructions; and a processor in communicationwith the memory. When the processor executes the instructions, theprocessor is configured to cause the apparatus to perform the abovemethods for video decoding and/or encoding.

According to yet another aspect, an embodiment of the present disclosureprovides non-transitory computer-readable mediums storing instructionswhich when executed by a computer for video decoding and/or encodingcause the computer to perform the above methods for video decodingand/or encoding.

The above and other aspects and their implementations are described ingreater detail in the drawings, the descriptions, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A shows a schematic illustration of an exemplary subset of intraprediction directional modes.

FIG. 1B shows an illustration of exemplary intra prediction directions.

FIG. 2 shows a schematic illustration of a current block and itssurrounding spatial merge candidates for motion vector prediction in oneexample.

FIG. 3 shows a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an example embodiment.

FIG. 4 shows a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an example embodiment.

FIG. 5 shows a schematic illustration of a simplified block diagram of avideo decoder in accordance with an example embodiment.

FIG. 6 shows a schematic illustration of a simplified block diagram of avideo encoder in accordance with an example embodiment.

FIG. 7 shows a block diagram of a video encoder in accordance withanother example embodiment.

FIG. 8 shows a block diagram of a video decoder in accordance withanother example embodiment.

FIG. 9 shows directional intra prediction modes according to exampleembodiments of the disclosure.

FIG. 10 shows non-directional intra prediction modes according toexample embodiments of the disclosure.

FIG. 11 shows recursive intra prediction modes according to exampleembodiments of the disclosure.

FIG. 12 shows transform block partitioning and scan of an intraprediction block according to example embodiments of the disclosure.

FIG. 13 shows transform block partitioning and scan of an interprediction block according to example embodiments of the disclosure.

FIG. 14 shows low frequency non-separable transform process according toexample embodiments of the disclosure.

FIG. 15 shows a data flow for performing non-separable transformaccording to example embodiments of the disclosure.

FIG. 16 shows a flow chart according to example embodiments of thedisclosure.

FIG. 17 shows a schematic illustration of a computer system inaccordance with example embodiments of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the example of FIG. 3 , the first pair of terminal devices (310) and(320) may perform unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., of a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display the video pictures according tothe recovered video data. Unidirectional data transmission may beimplemented in media serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that perform bidirectionaltransmission of coded video data that may be implemented, for example,during a videoconferencing application. For bidirectional transmissionof data, in an example, each terminal device of the terminal devices(330) and (340) may code video data (e.g., of a stream of video picturesthat are captured by the terminal device) for transmission to the otherterminal device of the terminal devices (330) and (340) via the network(350). Each terminal device of the terminal devices (330) and (340) alsomay receive the coded video data transmitted by the other terminaldevice of the terminal devices (330) and (340), and may decode the codedvideo data to recover the video pictures and may display the videopictures at an accessible display device according to the recoveredvideo data.

In the example of FIG. 3 , the terminal devices (310), (320), (330) and(340) may be implemented as servers, personal computers and smart phonesbut the applicability of the underlying principles of the presentdisclosure may not be so limited. Embodiments of the present disclosuremay be implemented in desktop computers, laptop computers, tabletcomputers, media players, wearable computers, dedicated videoconferencing equipment, and/or the like. The network (350) representsany number or types of networks that convey coded video data among theterminal devices (310), (320), (330) and (340), including for examplewireline (wired) and/or wireless communication networks. Thecommunication network (350)9 may exchange data in circuit-switched,packet-switched, and/or other types of channels. Representative networksinclude telecommunications networks, local area networks, wide areanetworks and/or the Internet. For the purposes of the presentdiscussion, the architecture and topology of the network (350) may beimmaterial to the operation of the present disclosure unless explicitlyexplained herein.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, a placement of a video encoder and a video decoder in avideo streaming environment. The disclosed subject matter may be equallyapplicable to other video applications, including, for example, videoconferencing, digital TV broadcasting, gaming, virtual reality, storageof compressed video on digital media including CD, DVD, memory stick andthe like, and so on.

A video streaming system may include a video capture subsystem (413)that can include a video source (401), e.g., a digital camera, forcreating a stream of video pictures or images (402) that areuncompressed. In an example, the stream of video pictures (402) includessamples that are recorded by a digital camera of the video source 401.The stream of video pictures (402), depicted as a bold line to emphasizea high data volume when compared to encoded video data (404) (or codedvideo bitstreams), can be processed by an electronic device (420) thatincludes a video encoder (403) coupled to the video source (401). Thevideo encoder (403) can include hardware, software, or a combinationthereof to enable or implement aspects of the disclosed subject matteras described in more detail below. The encoded video data (404) (orencoded video bitstream (404)), depicted as a thin line to emphasize alower data volume when compared to the stream of uncompressed videopictures (402), can be stored on a streaming server (405) for future useor directly to downstream video devices (not shown). One or morestreaming client subsystems, such as client subsystems (406) and (408)in FIG. 4 can access the streaming server (405) to retrieve copies (407)and (409) of the encoded video data (404). A client subsystem (406) caninclude a video decoder (410), for example, in an electronic device(430). The video decoder (410) decodes the incoming copy (407) of theencoded video data and creates an outgoing stream of video pictures(411) that are uncompressed and that can be rendered on a display (412)(e.g., a display screen) or other rendering devices (not depicted). Thevideo decoder 410 may be configured to perform some or all of thevarious functions described in this disclosure. In some streamingsystems, the encoded video data (404), (407), and (409) (e.g., videobitstreams) can be encoded according to certain video coding/compressionstandards. Examples of those standards include ITU-T RecommendationH.265. In an example, a video coding standard under development isinformally known as Versatile Video Coding (VVC). The disclosed subjectmatter may be used in the context of VVC, and other video codingstandards.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anyembodiment of the present disclosure below. The video decoder (510) canbe included in an electronic device (530). The electronic device (530)can include a receiver (531) (e.g., receiving circuitry). The videodecoder (510) can be used in place of the video decoder (410) in theexample of FIG. 4 .

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510). In the same or another embodiment,one coded video sequence may be decoded at a time, where the decoding ofeach coded video sequence is independent from other coded videosequences. Each video sequence may be associated with multiple videoframes or images. The coded video sequence may be received from achannel (501), which may be a hardware/software link to a storage devicewhich stores the encoded video data or a streaming source whichtransmits the encoded video data. The receiver (531) may receive theencoded video data with other data such as coded audio data and/orancillary data streams, that may be forwarded to their respectiveprocessing circuitry (not depicted). The receiver (531) may separate thecoded video sequence from the other data. To combat network jitter, abuffer memory (515) may be disposed in between the receiver (531) and anentropy decoder/parser (520) (“parser (520)” henceforth). In certainapplications, the buffer memory (515) may be implemented as part of thevideo decoder (510). In other applications, it can be outside of andseparate from the video decoder (510) (not depicted). In still otherapplications, there can be a buffer memory (not depicted) outside of thevideo decoder (510) for the purpose of, for example, combating networkjitter, and there may be another additional buffer memory (515) insidethe video decoder (510), for example to handle playback timing. When thereceiver (531) is receiving data from a store/forward device ofsufficient bandwidth and controllability, or from an isosynchronousnetwork, the buffer memory (515) may not be needed, or can be small. Foruse on best-effort packet networks such as the Internet, the buffermemory (515) of sufficient size may be required, and its size can becomparatively large. Such buffer memory may be implemented with anadaptive size, and may at least partially be implemented in an operatingsystem or similar elements (not depicted) outside of the video decoder(510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such asdisplay (512) (e.g., a display screen) that may or may not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as is shown in FIG. 5 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received by theparser (520). The entropy coding of the coded video sequence can be inaccordance with a video coding technology or standard, and can followvarious principles, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (520) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameter corresponding to thesubgroups. The subgroups can include Groups of Pictures (GOPs),pictures, tiles, slices, macroblocks, Coding Units (CUs), blocks,Transform Units (TUs), Prediction Units (PUs) and so forth. The parser(520) may also extract from the coded video sequence information such astransform coefficients (e.g., Fourier transform coefficients), quantizerparameter values, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple differentprocessing or functional units depending on the type of the coded videopicture or parts thereof (such as: inter and intra picture, inter andintra block), and other factors. The units that are involved and howthey are involved may be controlled by the subgroup control informationthat was parsed from the coded video sequence by the parser (520). Theflow of such subgroup control information between the parser (520) andthe multiple processing or functional units below is not depicted forsimplicity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these functional units interact closelywith each other and can, at least partly, be integrated with oneanother. However, for the purpose of describing the various functions ofthe disclosed subject matter with clarity, the conceptual subdivisioninto the functional units is adopted in the disclosure below.

A first unit may include the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) may receive a quantized transformcoefficient as well as control information, including informationindicating which type of inverse transform to use, block size,quantization factor/parameters, quantization scaling matrices, and thelie as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values that canbe input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block, i.e., a block that does not usepredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) may generate a block of the same size and shape ofthe block under reconstruction using surrounding block information thatis already reconstructed and stored in the current picture buffer (558).The current picture buffer (558) buffers, for example, partlyreconstructed current picture and/or fully reconstructed currentpicture. The aggregator (555), in some implementations, may add, on aper sample basis, the prediction information the intra prediction unit(552) has generated to the output sample information as provided by thescaler/inverse transform unit (551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forinter-picture prediction. After motion compensating the fetched samplesin accordance with the symbols (521) pertaining to the block, thesesamples can be added by the aggregator (555) to the output of thescaler/inverse transform unit (551) (output of unit 551 may be referredto as the residual samples or residual signal) so as to generate outputsample information. The addresses within the reference picture memory(557) from where the motion compensation prediction unit (553) fetchesprediction samples can be controlled by motion vectors, available to themotion compensation prediction unit (553) in the form of symbols (521)that can have, for example X, Y components (shift), and referencepicture components (time). Motion compensation may also includeinterpolation of sample values as fetched from the reference picturememory (557) when sub-sample exact motion vectors are in use, and mayalso be associated with motion vector prediction mechanisms, and soforth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values. Several type of loop filters may beincluded as part of the loop filter unit 556 in various orders, as willbe described in further detail below.

The output of the loop filter unit (556) can be a sample stream that canbe output to the rendering device (512) as well as stored in thereference picture memory (557) for use in future inter-pictureprediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future inter-picture prediction. For example,once a coded picture corresponding to a current picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, the parser (520)), the current picture buffer(558) can become a part of the reference picture memory (557), and afresh current picture buffer can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology adopted in a standard, suchas ITU-T Rec. H.265. The coded video sequence may conform to a syntaxspecified by the video compression technology or standard being used, inthe sense that the coded video sequence adheres to both the syntax ofthe video compression technology or standard and the profiles asdocumented in the video compression technology or standard.Specifically, a profile can select certain tools from all the toolsavailable in the video compression technology or standard as the onlytools available for use under that profile. To be standard-compliant,the complexity of the coded video sequence may be within bounds asdefined by the level of the video compression technology or standard. Insome cases, levels restrict the maximum picture size, maximum framerate, maximum reconstruction sample rate (measured in, for examplemegasamples per second), maximum reference picture size, and so on.Limits set by levels can, in some cases, be further restricted throughHypothetical Reference Decoder (HRD) specifications and metadata for HRDbuffer management signaled in the coded video sequence.

In some example embodiments, the receiver (531) may receive additional(redundant) data with the encoded video. The additional data may beincluded as part of the coded video sequence(s). The additional data maybe used by the video decoder (510) to properly decode the data and/or tomore accurately reconstruct the original video data. Additional data canbe in the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anexample embodiment of the present disclosure. The video encoder (603)may be included in an electronic device (620). The electronic device(620) may further include a transmitter (640) (e.g., transmittingcircuitry). The video encoder (603) can be used in place of the videoencoder (403) in the example of FIG. 4 .

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the example ofFIG. 6 ) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) may beimplemented as a portion of the electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 YCrCb, RGB, XYZ . . .), and any suitable sampling structure (for example YCrCb 4:2:0, YCrCb4:4:4). In a media serving system, the video source (601) may be astorage device capable of storing previously prepared video. In avideoconferencing system, the video source (601) may be a camera thatcaptures local image information as a video sequence. Video data may beprovided as a plurality of individual pictures or images that impartmotion when viewed in sequence. The pictures themselves may be organizedas a spatial array of pixels, wherein each pixel can comprise one ormore samples depending on the sampling structure, color space, and thelike being in use. A person having ordinary skill in the art can readilyunderstand the relationship between pixels and samples. The descriptionbelow focuses on samples.

According to some example embodiments, the video encoder (603) may codeand compress the pictures of the source video sequence into a codedvideo sequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speedconstitutes one function of a controller (650). In some embodiments, thecontroller (650) may be functionally coupled to and control otherfunctional units as described below. The coupling is not depicted forsimplicity. Parameters set by the controller (650) can include ratecontrol related parameters (picture skip, quantizer, lambda value ofrate-distortion optimization techniques, . . . ), picture size, group ofpictures (GOP) layout, maximum motion vector search range, and the like.The controller (650) can be configured to have other suitable functionsthat pertain to the video encoder (603) optimized for a certain systemdesign.

In some example embodiments, the video encoder (603) may be configuredto operate in a coding loop. As an oversimplified description, in anexample, the coding loop can include a source coder (630) (e.g.,responsible for creating symbols, such as a symbol stream, based on aninput picture to be coded, and a reference picture(s)), and a (local)decoder (633) embedded in the video encoder (603). The decoder (633)reconstructs the symbols to create the sample data in a similar manneras a (remote) decoder would create even though the embedded decoder 633process coded video steam by the source coder 630 without entropy coding(as any compression between symbols and coded video bitstream in entropycoding may be lossless in the video compression technologies consideredin the disclosed subject matter). The reconstructed sample stream(sample data) is input to the reference picture memory (634). As thedecoding of a symbol stream leads to bit-exact results independent ofdecoder location (local or remote), the content in the reference picturememory (634) is also bit exact between the local encoder and remoteencoder. In other words, the prediction part of an encoder “sees” asreference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used to improve coding quality.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5 . Brieflyreferring also to FIG. 5 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633) inthe encoder.

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that may only be presentin a decoder also may necessarily need to be present, in substantiallyidentical functional form, in a corresponding encoder. For this reason,the disclosed subject matter may at times focus on decoder operation,which allies to the decoding portion of the encoder. The description ofencoder technologies can thus be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas or aspects a more detail description of the encoder is providedbelow.

During operation in some example implementations, the source coder (630)may perform motion compensated predictive coding, which codes an inputpicture predictively with reference to one or more previously codedpicture from the video sequence that were designated as “referencepictures.” In this manner, the coding engine (632) codes differences (orresidue) in the color channels between pixel blocks of an input pictureand pixel blocks of reference picture(s) that may be selected asprediction reference(s) to the input picture. The term “residue” and itsadjective form “residual” may be used interchangeably.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end (remote) video decoder (absent transmissionerrors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compression of the symbols accordingto technologies such as Huffman coding, variable length coding,arithmetic coding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person having ordinary skill in the art is aware of thosevariants of I pictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample coding blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16samples each) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures. The sourcepictures or the intermediate processed pictures may be subdivided intoother types of blocks for other purposes. The division of coding blocksand the other types of blocks may or may not follow the same manner, asdescribed in further detail below.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data may accordingly conform to a syntax specified bythe video coding technology or standard being used.

In some example embodiments, the transmitter (640) may transmitadditional data with the encoded video. The source coder (630) mayinclude such data as part of the coded video sequence. The additionaldata may comprise temporal/spatial/SNR enhancement layers, other formsof redundant data such as redundant pictures and slices, SEI messages,VUI parameter set fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) utilizes spatial correlation in a givenpicture, and inter-picture prediction utilizes temporal or othercorrelation between the pictures. For example, a specific picture underencoding/decoding, which is referred to as a current picture, may bepartitioned into blocks. A block in the current picture, when similar toa reference block in a previously coded and still buffered referencepicture in the video, may be coded by a vector that is referred to as amotion vector. The motion vector points to the reference block in thereference picture, and can have a third dimension identifying thereference picture, in case multiple reference pictures are in use.

In some example embodiments, a bi-prediction technique can be used forinter-picture prediction. According to such bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that both proceed the current picture in the video indecoding order (but may be in the past or future, respectively, indisplay order) are used. A block in the current picture can be coded bya first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bejointly predicted by a combination of the first reference block and thesecond reference block.

Further, a merge mode technique may be used in the inter-pictureprediction to improve coding efficiency.

According to some example embodiments of the disclosure, predictions,such as inter-picture predictions and intra-picture predictions areperformed in the unit of blocks. For example, a picture in a sequence ofvideo pictures is partitioned into coding tree units (CTU) forcompression, the CTUs in a picture may have the same size, such as 64×64pixels, 32×32 pixels, or 16×16 pixels. In general, a CTU may includethree parallel coding tree blocks (CTBs): one luma CTB and two chromaCTBs. Each CTU can be recursively quadtree split into one or multiplecoding units (CUs). For example, a CTU of 64×64 pixels can be split intoone CU of 64×64 pixels, or 4 CUs of 32×32 pixels. Each of the one ormore of the 32×32 block may be further split into 4 CUs of 16×16 pixels.In some example embodiments, each CU may be analyzed during encoding todetermine a prediction type for the CU among various prediction typessuch as an inter prediction type or an intra prediction type. The CU maybe split into one or more prediction units (PUs) depending on thetemporal and/or spatial predictability. Generally, each PU includes aluma prediction block (PB), and two chroma PBs. In an embodiment, aprediction operation in coding (encoding/decoding) is performed in theunit of a prediction block. The split of a CU into PU (or PBs ofdifferent color channels) may be performed in various spatial pattern. Aluma or chroma PB, for example, may include a matrix of values (e.g.,luma values) for samples, such as 8×8 pixels, 16×16 pixels, 8×16 pixels,16×8 samples, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherexample embodiment of the disclosure. The video encoder (703) isconfigured to receive a processing block (e.g., a prediction block) ofsample values within a current video picture in a sequence of videopictures, and encode the processing block into a coded picture that ispart of a coded video sequence. The example video encoder (703) may beused in place of the video encoder (403) in the FIG. 4 example.

For example, the video encoder (703) receives a matrix of sample valuesfor a processing block, such as a prediction block of 8×8 samples, andthe like. The video encoder (703) then determines whether the processingblock is best coded using intra mode, inter mode, or bi-prediction modeusing, for example, rate-distortion optimization (RDO). When theprocessing block is determined to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis determined to be coded in inter mode or bi-prediction mode, the videoencoder (703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Insome example embodiments, a merge mode may be used as a submode of theinter picture prediction where the motion vector is derived from one ormore motion vector predictors without the benefit of a coded motionvector component outside the predictors. In some other exampleembodiments, a motion vector component applicable to the subject blockmay be present. Accordingly, the video encoder (703) may includecomponents not explicitly shown in FIG. 7 , such as a mode decisionmodule, to determine the perdition mode of the processing blocks.

In the example of FIG. 7 , the video encoder (703) includes an interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in the examplearrangement in FIG. 7 .

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures in display order), generate inter predictioninformation (e.g., description of redundant information according tointer encoding technique, motion vectors, merge mode information), andcalculate inter prediction results (e.g., predicted block) based on theinter prediction information using any suitable technique. In someexamples, the reference pictures are decoded reference pictures that aredecoded based on the encoded video information using the decoding unit633 embedded in the example encoder 620 of FIG. 6 (shown as residualdecoder 728 of FIG. 7 , as described in further detail below).

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to blocksalready coded in the same picture, and generate quantized coefficientsafter transform, and in some cases also to generate intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). The intra encoder (722) maycalculates intra prediction results (e.g., predicted block) based on theintra prediction information and reference blocks in the same picture.

The general controller (721) may be configured to determine generalcontrol data and control other components of the video encoder (703)based on the general control data. In an example, the general controller(721) determines the prediction mode of the block, and provides acontrol signal to the switch (726) based on the prediction mode. Forexample, when the prediction mode is the intra mode, the generalcontroller (721) controls the switch (726) to select the intra moderesult for use by the residue calculator (723), and controls the entropyencoder (725) to select the intra prediction information and include theintra prediction information in the bitstream; and when the predicationmode for the block is the inter mode, the general controller (721)controls the switch (726) to select the inter prediction result for useby the residue calculator (723), and controls the entropy encoder (725)to select the inter prediction information and include the interprediction information in the bitstream.

The residue calculator (723) may be configured to calculate a difference(residue data) between the received block and prediction results for theblock selected from the intra encoder (722) or the inter encoder (730).The residue encoder (724) may be configured to encode the residue datato generate transform coefficients. For example, the residue encoder(724) may be configured to convert the residue data from a spatialdomain to a frequency domain to generate the transform coefficients. Thetransform coefficients are then subject to quantization processing toobtain quantized transform coefficients. In various example embodiments,the video encoder (703) also includes a residual decoder (728). Theresidual decoder (728) is configured to perform inverse-transform, andgenerate the decoded residue data. The decoded residue data can besuitably used by the intra encoder (722) and the inter encoder (730).For example, the inter encoder (730) can generate decoded blocks basedon the decoded residue data and inter prediction information, and theintra encoder (722) can generate decoded blocks based on the decodedresidue data and the intra prediction information. The decoded blocksare suitably processed to generate decoded pictures and the decodedpictures can be buffered in a memory circuit (not shown) and used asreference pictures.

The entropy encoder (725) may be configured to format the bitstream toinclude the encoded block and perform entropy coding. The entropyencoder (725) is configured to include in the bitstream variousinformation. For example, the entropy encoder (725) may be configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. When coding a block in the merge submode of either inter modeor bi-prediction mode, there may be no residue information.

FIG. 8 shows a diagram of an example video decoder (810) according toanother embodiment of the disclosure. The video decoder (810) isconfigured to receive coded pictures that are part of a coded videosequence, and decode the coded pictures to generate reconstructedpictures. In an example, the video decoder (810) may be used in place ofthe video decoder (410) in the example of FIG. 4 .

In the example of FIG. 8 , the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residual decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in the example arrangement of FIG. 8 .

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (e.g., intra mode, intermode, bi-predicted mode, merge submode or another submode), predictioninformation (e.g., intra prediction information or inter predictioninformation) that can identify certain sample or metadata used forprediction by the intra decoder (872) or the inter decoder (880),residual information in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isthe inter or bi-predicted mode, the inter prediction information isprovided to the inter decoder (880); and when the prediction type is theintra prediction type, the intra prediction information is provided tothe intra decoder (872). The residual information can be subject toinverse quantization and is provided to the residual decoder (873).

The inter decoder (880) may be configured to receive the interprediction information, and generate inter prediction results based onthe inter prediction information.

The intra decoder (872) may be configured to receive the intraprediction information, and generate prediction results based on theintra prediction information.

The residual decoder (873) may be configured to perform inversequantization to extract de-quantized transform coefficients, and processthe de-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residual decoder (873) mayalso utilize certain control information (to include the QuantizerParameter (QP)) which may be provided by the entropy decoder (871) (datapath not depicted as this may be low data volume control informationonly).

The reconstruction module (874) may be configured to combine, in thespatial domain, the residual as output by the residual decoder (873) andthe prediction results (as output by the inter or intra predictionmodules as the case may be) to form a reconstructed block forming partof the reconstructed picture as part of the reconstructed video. It isnoted that other suitable operations, such as a deblocking operation andthe like, may also be performed to improve the visual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In some example embodiments, the video encoders(403), (603), and (703), and the video decoders (410), (510), and (810)can be implemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Returning to the intra prediction process, in which samples in a block(e.g., a luma or chroma prediction block, or coding block if not furthersplit into prediction blocks) is predicted by samples of neighboring,next neighboring, or other line or lines, or the combination thereof, togenerate a prediction block. The residual between the actual block beingcoded and the prediction block may then be processed via transformfollowed by quantization. Various intra prediction modes may be madeavailable and parameters related to intra mode selection and otherparameters may be signaled in the bitstream. The various intraprediction modes, for example, may pertain to line position or positionsfor predicting samples, directions along which prediction samples areselected from predicting line or lines, and other special intraprediction modes.

For example, a set of intra prediction modes (interchangeably referredto as “intra modes”) may include a predefined number of directionalintra prediction modes. As described above in relation to the exampleimplementation of FIG. 1 , these intra prediction modes may correspondto a predefined number of directions along which out-of-block samplesare selected as prediction for samples being predicted in a particularblock. In another particular example implementation, eight (8) maindirectional modes corresponding to angles from 45 to 207 degrees to thehorizontal axis may be supported and predefined.

In some other implementations of intra prediction, to further exploitmore varieties of spatial redundancy in directional textures,directional intra modes may be further extended to an angle set withfiner granularity. For example, the 8-angle implementation above may beconfigured to provide eight nominal angles, referred to as V_PRED,H_PRED, D45_PRED, D135_PRED, D113_PRED, D157_PRED, D203_PRED, andD67_PRED, as illustrated in FIG. 9 , and for each nominal angle, apredefined number (e.g., 7) of finer angles may be added. With such anextension, a larger total number (e.g., 56 in this example) ofdirectional angles may be available for intra prediction, correspondingto the same number of predefined directional intra modes. A predictionangle may be represented by a nominal intra angle plus an angle delta.For the particular example above with 7 finer angular directions foreach nominal angle, the angle delta may be −3˜3 multiplies a step sizeof 3 degrees.

In some implementations, alternative or in addition to the directionintra modes above, a predefined number of non-directional intraprediction modes may also be predefined and made available. For example,5 non-direction intra modes referred to as smooth intra prediction modesmay be specified. These non-directional intra mode prediction modes maybe specifically referred to as DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_Hintra modes. Prediction of samples of a particular block under theseexample non-directional modes are illustrated in FIG. 10 . As anexample, FIG. 10 shows a 4×4 block 1002 being predicted by samples froma top neighboring line and/or left neighboring line. A particular sample1010 in block 1002 may correspond to directly top sample 1004 of thesample 1010 in the top neighboring line of block 1002, a top-left sample1006 of the sample 1010 as the intersection of the top and leftneighboring lines, and a directly left sample 1008 of the sample 1010 inthe left neighboring line of block 1002. For the example DC intraprediction mode, an average of the left and above neighboring samples1008 and 1004 may be used as the predictor of the sample 1010. For theexample PAETH intra prediction mode, the top, left, and top-leftreference samples 1004, 1008, and 1006 may be fetched, and thenwhichever value among these three reference samples that is the closestto (top+left−topleft) may be set as the predictor for the sample 1010.For the example SMOOTH_V intra prediction mode, the sample 1010 may bepredicted by a quadratic interpolation in vertical direction of thetop-left neighboring sample 1006 and the left neighboring sample 1008.For the example SMOOTH_H intra prediction mode, the sample 1010 may bepredicted by a quadratic interpolation in horizontal direction of thetop-left neighboring sample 1006 and the top neighboring sample 1004.For the example SMOOTH intra prediction mode, the sample 1010 may bepredicted by an average of the quadratic interpolations in the verticaland the horizontal directions. The non-directional intra modeimplementations above are merely illustrated as a non-limiting example.Other neighboring lines, and other non-directional selection of samples,and manners of combining predicting samples for predicting a particularsample in a prediction block are also contemplated.

Selection of a particular intra prediction mode by the encoder from thedirectional or non-directional modes above at various coding levels(picture, slice, block, unit, etc.) may be signaled in the bitstream. Insome example implementations, the exemplary 8 nominal directional modestogether with 5 non-angular smooth modes (a total of 13 options) may besignaled first. Then if the signaled mode is one of the 8 nominalangular intra modes, an index is further signaled to indicate theselected angle delta to the corresponding signaled nominal angle. Insome other example implementations, all intra prediction modes may beindexed all together (e.g., 56 directional modes plus 5 non-directionalmodes to yield 61 intra prediction modes) for signaling.

In some example implementations, the example 56 or other number ofdirectional intra prediction modes may be implemented with a unifieddirectional predictor that projects each sample of a block to areference sub-sample location and interpolates the reference sample by a2-tap bilinear filter.

In some implementations, to capture decaying spatial correlation withreferences on the edges, additional filter modes referred to as FILTERINTRA modes may be designed. For these modes, predicted samples withinthe block in addition to out-of-block samples may be used as intraprediction reference samples for some patches within the block. Thesemodes, for example, may be predefined and made available to intraprediction for at least luma blocks (or only luma blocks). A predefinednumber (e.g., five) of filter intra modes may be pre-designed, eachrepresented by a set of n-tap filters (e.g., 7-tap filters) reflectingcorrelation between samples in, for example, a 4×2 patch and n neighborsadjacent to it. In other words, the weighting factors for an n-tapfilter may be position dependent. Taking an 8×8 block, 4×2 patch, and7-tap filtering as an example, as shown in FIG. 11 , the 8×8 block 1102may be split into eight 4×2 patches. These patches are indicated by B0,B1, B1, B3, B4, B5, B6, and B7 in FIG. 11 . For each patch, its 7neighbors, indicated by R0˜R7 in FIG. 11 , may be used to predict thesamples in a current patch. For patch B0, all the neighbors may havebeen already reconstructed. But for other patches, some of the neighborsare in the current block and thus may not have been reconstructed, thenthe predicted values of immediate neighbors are used as the reference.For example, all the neighbors of patch B7 as indicated in FIG. 11 arenot reconstructed, so the prediction samples of neighbors are usedinstead.

In some implementation of intra prediction, one color component may bepredicted using one or more other color components. A color componentmay be any one of components in YCrCb, RGB, XYZ color space and thelike. For example, a prediction of chroma component (e.g., chroma block)from luma component (e.g., luma reference samples), referred to asChroma from Luma, or CfL), may be implemented. In some exampleimplementations, cross-color prediction many only be allowed from lumato chroma. For example, a chroma sample in a chroma block may be modeledas a linear function of coincident reconstructed luma samples. The CfLprediction may be implemented as follows:

CfL(α)=α×L ^(AC) +DC  (1)

where L^(AC) denotes an AC contribution of luma component, a denotes aparameter of the linear model, and DC denotes a DC contribution of thechroma component. The AC components, for example is obtained for eachsamples of the block whereas the DC component is obtained for the entireblock. To be specific, the reconstructed luma samples may be subsampledinto the chroma resolution, and then the average luma value (DC of luma)may be subtracted from each luma value to form the AC contribution inluma. The AC contribution of Luma is then used in the linear mode of Eq.(1) to predict the AC values of the chroma component. To approximate orpredict chroma AC component from the luma AC contribution, instead ofrequiring the decoder to calculate the scaling parameters, an exampleCfL implementation may determine the parameter a based on the originalchroma samples and signal them in the bitstream. This reduces decodercomplexity and yields more precise predictions. As for the DCcontribution of the chroma component, it may be computed using intra DCmode within the chroma component in some example implementations.

Transform of a residual of either an intra prediction block or an interprediction block may then be implemented followed by quantization of thetransform coefficient. For the purpose of performing transform, bothintra and inter coded blocks may be further partitioned into multipletransform blocks (sometimes interchangeably used as “transform units”,even though the term “unit” is normally used to represent a congregationof the three-color channels, e.g., a “coding unit” would include lumacoding block, and chroma coding blocks) prior to the transform. In someimplementations, the maximum partitioning depth of the coded blocks (orprediction blocks) may be specified (the term “coded blocks” may be usedinterchangeably with “coding blocks”). For example, such partitioningmay not go beyond 2 levels. The division of prediction block intotransform blocks may be handled differently between intra predictionblocks and inter prediction blocks. In some implementations, however,such division may be similar between intra prediction blocks and interprediction blocks.

In some example implementations, and for intra coded blocks, thetransform partition may be done in a way that all the transform blockshave the same size, and the transform blocks are coded in a raster scanorder. An example of such transform block partitioning of an intra codedblock is shown in FIG. 12 . Specifically, FIG. 12 illustrates the codedblock 1202 is partitioned via an intermediate level quadtree splitting1204 into 16 transform blocks of the same block size, as shown by 1206.The example raster scan order for coding is illustrated by the orderedarrows in FIG. 12 .

In some example implementations, and for inter coded blocks, thetransform unit partitioning may be done in a recursive manner with thepartitioning depth up to a predefined number of levels (e.g., 2 levels).Split may stop or continue recursively for any sub partition and at anylevel, as shown in FIG. 13 . In particular, FIG. 13 shows an examplewhere the block 1302 is split into four quadtree sub blocks 1304 and oneof the subblocks is further split into four second level transformblocks whereas division of the other subblocks stops after the firstlevel, yielding a total of 7 transform blocks of two different sizes.The example raster scan order for coding is further illustrated by theordered arrows in FIG. 13 . While FIG. 13 shows an exampleimplementation of quadtree split of up-to two levels of square transformblocks, in some generation implementations, the transform partitioningmay support 1:1 (square), 1:2/2:1, and 1:4/4:1 transform block shapesand sizes ranging from 4×4 to 64×64. In some example implementations, ifthe coding block is smaller than or equal to 64×64, the transform blockpartitioning may only be applied to luma component (in other words, thechroma transform block would be the same as the coding block under thatcondition). Otherwise, if the coding block width or height is greaterthan 64, both the luma and chroma coding blocks may be implicitly splitinto multiples of min (W, 64)×min (H, 64) and min (W, 32)×min (H, 32)transform blocks, respectively.

Each of the transform blocks above may then be subject to a primarytransform. The primary transform essentially moves the residual in atransform block from spatial domain to frequency domain. In someimplementation of the actual primary transform, in order to support theexample extended coding block partitions above, multiple transform sizes(ranging from 4-point to 64-point for each dimension of the twodimensions) and transform shapes (square; rectangular with width/heightratio's 2:1/1:2, and 4:1/1:4) may be allowed.

Turning to the actual primary transform, in some exampleimplementations, a 2-D transform process may involve a use of hybridtransform kernels (which, for example, may be composed of different 1-Dtransforms for each dimension of the coded residual transform block).Example 1-D transform kernels may include but are not limited to: a)4-point, 8-point, 16-point, 32-point, 64-point DCT-2; b) 4-point,8-point, 16-point asymmetric DST's (DST-4, DST-7) and their flippedversions; c) 4-point, 8-point, 16-point, 32-point identity transforms.Selection of transform kernels to be used for each dimension may bebased on a rate-distortion (RD) criterion. For example, the basisfunctions for the DCT-2 and asymmetric DST's that may be implemented arelisted in Table 1.

TABLE 1 Example primary transform basis functions (DCT-2, DST-4 andDST-7 for N-point input). Transform Type Basis function T_(i)(j), i, j =0, 1, . . . , N − 1 DCT-2${T_{i}(j)} = {\omega_{0} \cdot \sqrt{\frac{2}{N}} \cdot {\cos( \frac{\pi \cdot i \cdot ( {{2j} + 1} )}{2N} )}}$${{where}\omega_{0}} = \{ \begin{matrix}\sqrt{\frac{2}{N}} & {i = 0} \\1 & {i \neq 0}\end{matrix} $ DST-4${T_{i}(j)} = {\sqrt{\frac{2}{N}} \cdot {\sin( \frac{\pi \cdot ( {{2i} + 1} ) \cdot ( {{2j} + 1} )}{4N} )}}$DST-7${T_{i}(j)} = {\sqrt{\frac{4}{{2N} + 1}} \cdot {\sin( \frac{\pi \cdot ( {{2i} + 1} ) \cdot ( {j + 1} )}{{2N} + 1} )}}$

In some example implementations, the availability of hybrid transformkernels for a particular primary transform implementation may be basedon the transform block size and prediction mode. An example dependencyis listed in Table 2. For a chroma component, the transform typeselection may be performed in an implicit way. For example, for intraprediction residuals, the transform type may be selected according tothe intra prediction mode, as specified in Table 3. For inter predictionresiduals, the transform type for chroma blocks may be selectedaccording to the transform type selection of the co-located luma blocks.Therefore, for chroma component, there is no transform type signaling inthe bitstream.

TABLE 2 AV1 hybrid transform kernels and their availability based onprediction modes and block sizes. Here → and ↓ denote the horizontal andvertical dimensions; ✓ and x denotes the availability of a kernel forthat block size & prediction mode. Prediction mode Transform TypesDescription Intra Inter DCT_DCT DCT ↓ and → ✓ (all ✓ (all block blocksizes) sizes) ADST_DCT ADST ↓; DCT → ✓ ✓ DCT_ADST DCT ↓; ADST → (block(block ADST_ADST ADST ↓ and → size ≤ size ≤ 16 × 16) 16 × 16)FLIPADST_DCT FLIPADST ↓; x ✓ DCT → (block DCT_FLIPADST DCT ↓; size ≤FLIPADST → 16 × 16) FLIPADST_FLIPADST FLIPADST ↓ and → ADST_FLIPADSTADST ↓; FLIPADST → FLIPADST_ADST FLIPADST ↓; ADST → IDTX IDTX ↓ and → ✓✓ (block (block size ≤ size ≤ 16 × 16) 32 × 32) V_DCT DCT ↓; IDTX → ✓ ✓H_DCT IDTX ↓; DCT → (block (block size < size ≤ 16 × 16) 16 × 16) V_ADSTADST ↓; IDTX → x ✓ H_ADST IDTX ↓; ADST → (block size < 16 × 16)V_FLIPADST FLIPADST ↓; x ✓ IDTX → (block H_FLIPADST IDTX ↓; size <FLIPADST → 16 × 16)

TABLE 3 Transform type selection for chroma component intra predictionresiduals. Intra prediction Vertical Transform Horizontal TransformDC_PRED DCT DCT V_PRED ADST DCT H_PRED DCT ADST D45_PRED DCT DCTD135_PRED ADST ADST D113_PRED ADST DCT D157_PRED DCT ADST D203_PRED DCTADST D67_PRED ADST DCT SMOOTH_PRED ADST ADST SMOOTH_V_PRED ADST DCTSMOOTH_H_PRED DCT ADST PAETH_PRED ADST ADST

In some implementation, secondary transform on the primary transformcoefficients may be performed. For example, LFNST (low-frequencynon-separable transform), which is known as reduced secondary transformmay be applied between forward primary transform and quantization (atencoder) and between de-quantization and inverse primary transform (atdecoder side), as shown in FIG. 14 , to further decorrelate the primarytransform coefficients. In essence, LFNST may take a portion of theprimary transform coefficient, e.g., the low frequency portion (hence“reduced” from the full set of primary transform coefficients of thetransform block) to proceed to secondary transform. In an example LFNST,4×4 non-separable transform or 8×8 non-separable transform may beapplied according to transform block size. For example, 4×4 LFNST may beapplied for small transform blocks (e.g., min (width, height)<8) whereas8×8 LFNST may be applied for larger transform blocks (e.g., min (width,height)>8). For example, if an 8×8 transform block is subject to 4×4LFNST, then only the low frequency 4×4 portion of the 8×8 primarytransform coefficients is further undergo secondary transform.

As specifically shown in FIG. 14 , a transform block may be 8×8 (or16×16). Thus, the forward primary transform 1402 of the transform blockyields a 8×8 (or 16×16) primary transform coefficient matrix 1404, whereeach square unit represent a 2×2 (or 4×4) portion. The input to theforward LFNST, for example, may not be the entire 8×8 (or 16×16) primarytransform coefficients. For example, a 4×4 (or 8×8) LFNST may be usedfor secondary transform. As such, only the 4×4 (or 8×8) low frequencyprimary transform coeffects of the primary transform coefficient matrix1404, as indicated in the shaded portion (upper left) 1406 may be usedas input to the LFNST. The remaining portions of the primary transformcoefficient matrix may not be subject to secondary transform. As such,after the secondary transform, the portion of the primary transformcoeffects subject to the LFNST becomes the secondary transformcoefficients whereas the remaining portions not subject to LFNST (e.g.,the unshaded portions of the matrix 1404) maintain the correspondingprimary transform coefficients. In some example implementations, theremaining portion not subject of secondary transform may be all set tozero coefficient.

An example for application of a non-separable transform used in LFNST,is described below. To apply an example 4×4 LFNST, the 4×4 input block X(representing, e.g., the 4×4 low-frequency portion of a primarytransform coefficient block such as the shaded portion 1406 of theprimary transform matrix 1404 of FIG. 14 ) may be denoted as:

$\begin{matrix}{X = \begin{bmatrix}X_{00} & X_{01} & X_{02} & X_{03} \\X_{10} & X_{11} & X_{12} & X_{13} \\X_{20} & X_{21} & X_{22} & X_{23} \\X_{30} & X_{31} & X_{32} & X_{33}\end{bmatrix}} & (2)\end{matrix}$

This 2-D input matrix may be first linearized or scanned to a vector

in an example order:

=[X ₀₀ X ₀₁ X ₀₂ X ₀₃ X ₁₀ X ₁₁ X ₁₂ X ₁₃ X ₂₀ X ₂₁ X ₂₂ X ₂₃ X ₃₀ X ₃₁X ₃₂ X ₃₃]^(T)  (3)

The non-separable transform for the 4×4 LFNST may then be calculated as

=T·

, where

indicates the output transform coefficient vector, and Tis a 16×16transform matrix. The resulting 16×1 coefficient vector

is subsequently reverse scanned as 4×4 block using the scanning orderfor that block (e.g., horizontal, vertical or diagonal). Thecoefficients with smaller index may be placed with the smaller scanningindex in the 4×4 coefficient block. In such a manner, redundancy in aprimary transform coefficients X may be further exploit via the secondtransform T, thereby providing additional compression enhancement.

The example LFNST above is based on a direct matrix multiplicationapproach to apply non-separable transform so that it is implemented in asingle pass without multiple iterations. In some further exampleimplementations, the dimension of the non-separable transform matrix (T)for the example 4×4 LFNST may be further reduced to minimizecomputational complexity and memory space requirement for storing thetransform coefficients. Such implementations may be referred to asreduced non-separate transform (RST). In more detail, the main idea ofthe RST is to map an N (N is 4×4=16 in the example above, but may beequal to 64 for 8×8 blocks) dimensional vector to an R dimensionalvector in a different space, where N/R (R<N) represents the dimensionreduction factor. Hence, instead of N×N transform matrix, RST matrixbecomes an R×N matrix as follows:

$\begin{matrix}{T_{R \times N} = \begin{bmatrix}t_{11} & t_{12} & t_{13} & \ldots & t_{1N} \\t_{21} & t_{22} & t_{23} & \text{ } & t_{2N} \\\text{ } & \vdots & \text{ } & \ddots & \vdots \\t_{R1} & t_{R2} & t_{R3} & \ldots & t_{RN}\end{bmatrix}} & (4)\end{matrix}$

where the R rows of the transform matrix are reduced R basis of the Ndimensional space. The transformation thus converts an input vector or Ndimension to an output vector of reduced R dimension. As such, and asshown in FIG. 14 , the secondary transform coefficients 1408 transformedfrom the primary coefficients 1406 is reduced by a factor or N/R indimension. The three squares around 1408 in FIG. 14 may be zero-padded.

The inverse transform matrix for RTS may be the transpose of its forwardtransform. For an example 8×8 LFNST (contrasted with the 4×4 LFNSTabove, for a more diverse description here), an example reduction factorof 4 may be applied, and thus a 64×64 direct non-separable transformmatrix is accordingly reduced to 16×64 direct matrix. Further, in someimplementations, a portion rather than an entirety of the input primarycoefficients may be linearized into the input vector for the LFNST. Forexample, only a portion of the example 8×8 input primary transformcoefficients may be linearized into the X vector above. For a particularexample, out of the four 4×4 quadrants of the 8×8 primary transformcoefficient matrix, the bottom right (high frequency coefficients) maybe left out, and only the other three quadrants are linearized into a48×1 vector using a predefined scan order rather than a 64×1 vector. Insuch implementations, the non-separable transform matrix may be furtherreduced from 16×64 to 16×48.

Hence, an example reduced 48×16 inverse RST matrix may be used at thedecoder side to generate the top-left, top-right, and bottom-left 4×4quadrants of the 8×8 core (primary) transform coefficients.Specifically, when the further reduced 16×48 RST matrices are appliedinstead of the 16×64 RST with the same transform set configuration, thenon-separable secondary transformation would take as input thevectorized 48 matrix elements from three 4×4 quadrant blocks of the 8×8primary coefficient block excluding right-bottom 4×4 block. In suchimplementations the omitted right-bottom 4×4 primary transformcoefficient would be ignored in the secondary transformation. Thisfurther reduced transformation, would convert a vector of 48×1 into anoutput vector of 16×1, which is reverse scanned into a 4×4 matrix tofill 1408 of FIG. 14 . The three squares of the secondary transformcoefficients surrounding 1408 may be zero padded.

With the help of the such reduction of dimensions in the RST, memoryusage for storing all LFNST matrices is reduced. In the above example,the memory usage, for example, may be reduced from 10 KB to 8 KB withreasonably insignificant performance drop compared to the implementationwithout dimension reduction.

In some implementations, in order to reduce complexity, LFNST may befurther restricted to be applicable only if all coefficients outside theprimary transform coefficient portion to be subject to LFNST (e.g.,outside of the 1406 portion of 1404 in FIG. 14 ) are non-significant.Hence, all primary-only transform coefficients (e.g., the unshadedportion of the primary coefficient matrix 1404 of FIG. 4 ) may be nearzero when LFNST is applied. Such a restriction allows for a conditioningof the LFNST index signalling on the last-significant position, andhence avoids some extra coefficient scanning, which may be needed forchecking for significant coefficients at specific positions when thisrestriction is not applied. In some implementations, the worst-casehandling of LFNST (in terms of multiplications per pixel) may restrictthe non-separable transforms for 4×4 and 8×8 blocks to 8×16 and 8×48transforms, respectively. In those cases, the last-significant scanposition has to be less than 8, when LFNST is applied, for other sizesless than 16. For blocks with a shape of 4×N and N×4 and N>8, therestriction above implies that the LFNST is now applied only once to thetop-left 4×4 region only. As all primary-only coefficients are zero whenLFNST is applied, the number of operations needed for the primarytransforms is reduced in such cases. From an encoder perspective, thequantization of coefficients can be simplified when LFNST transforms aretested. A rate-distortion optimized quantization (RDO) has to be done atmaximum for the first 16 coefficients (in scan order), the remainingcoefficients may be enforced to be zero.

In some example implementations, the RST kernels available may bespecified as a number of transform sets with each transform setsincluding a number of non-separable transform matrices. For example,there may be a total of 4 transform sets and 2 non-separable transformmatrices (kernels) per transform set for used in LFNST. These kernelsmay be pre-trained offline and they are thus data driven. The offlinetrained transform kernels may be stored in memory or hard coded in anencoding or decoding device for use during encoding/decoding process. Aselection of the transform set during encoding or decoding process maybe determined by the intra prediction mode. A mapping from the intraprediction modes to the transform sets may be pre-defined. An example ofsuch predefined mapping is shown in Table 4. For example, as shown inTable 4, if one of three Cross-Component Linear Model (CCLM) modes(INTRA_LT_CCLM, INTRA_T_CCLM or INTRA_L_CCLM) is used for the currentblock (i.e., 81<=predModeIntra<=83), transform set 0 may be selected forthe current chroma block. For each transform set, the selectednon-separable secondary transform candidate may be further specified bythe explicitly signalled LFNST index. For example, the index may besignalled in a bit-stream once per intra CU after transformcoefficients.

TABLE 4 Transform selection table IntraPredMode Tr. set indexIntraPredMode < 0 1 0 <= IntraPredMode <= 1 0  2 <= IntraPredMode <= 121 13 <= IntraPredMode <= 23 2 24 <= IntraPredMode <= 44 3 45 <=IntraPredMode <= 55 2 56 <= IntraPredMode <= 80 1 81 <= IntraPredMode <=83 0

Because LFNST is restricted to be applicable only if all coefficientsoutside the first coefficient sub-group or portion are non-significantin the example implementations above, LFNST index coding depends on theposition of the last significant coefficient. In addition, the LFNSTindex may be context coded but does not depend on intra prediction mode,and only the first bin may be context coded. Furthermore, LFNST may beapplied for intra CU in both intra and inter slices, and for both lumaand chroma. If a dual tree is enabled, LFNST indices for Luma and Chromamay be signaled separately. For inter slice (the dual tree is disabled),a single LFNST index may be signaled and used for both Luma and Chroma.

In some example implementations, when Intra Sub-Partitioning (ISP) modeis selected, LFNST may be disabled and RST index may not be signaled,because performance improvement is likely to be marginal even if RST isapplied to every feasible partition block. Furthermore, disabling RSTfor ISP-predicted residual could reduce encoding complexity. In somefurther implementations, LFNST may also be disabled and the RST indexmay not be signaled when Multiple linear regression Intra Prediction(MIP) mode is selected.

Considering that a large CU greater than 64×64 (or any other predefinedsizes representing the maximum transform block size) is implicitly split(e.g., TU tiling) due to the existing maximum transform size restriction(e.g., 64×64), an LFNST index search could increase data buffering byfour times for a certain number of decode pipeline stages. Therefore, insome implementations, the maximum size that LFNST is allowed may berestricted to, for example, 64×64. In some implementations, LFNST may beenabled with DCT2 as primary transform only.

In some other implementations, intra secondary transform (IST) isprovided for luma component by defining, e.g., 12 sets of secondarytransforms, with, e.g., 3 kernels in each set. An intra mode dependentindex may be used for transform set selection. The kernel selectionwithin a set may be based on a signaled syntax element. The IST may beenabled when either DCT2 or ADST is used as both horizontal and verticalprimary transform. In some implementations, according to the block size,a 4×4 non-separable transform or 8×8 non-separable transform can beselected. If min (tx_width, tx_height)<8 the 4×4 IST can be selected.For larger blocks 8×8 IST can be used. Here tx_width & tx_heightcorrespond to transform block width & height, respectively. The input toIST may be low frequency primary transform coefficients in a zig-zagscan order.

The various transforms in video coding or decoding processes, e.g.,either primary transforms of samples in a residual blocks or secondarytransforms of blocks of primary transform coefficients processes may notbe every efficient in capturing directional texture patterns, such asedges which are 45-degree direction (e.g., directions that aresubstantially away from the horizontal or vertical directions), whenusing separable transform schemes only. As described above, in someexample implementations, one or more non-separable transform designs maybe used for secondary transform of primary transform coefficient. Asfurther described below, such non-separable transform schemes may alsobe employed for primary transform of residual sample blocks to generateprimary transform coefficients. In some example implementations, primarytransform coefficients, generated either via separable or non-separabletransforms, may be directly subject to quantization followed by entropycoding or alternatively may be subject to either separable ornon-separable secondary transforms before quantization and entropycoding. In implementations where primary transform coefficients obtainedvia non-separable transforms are further subject to non-separablesecondary transform, two non-separable transforms would have beenutilized in a cascading manner.

The disclosure below further describes some example implementations ofnon-separable transform schemes that apply to both primary and secondarytransforms. These non-separable transform designs described below aimfor improvement of coding efficiency, especially for directional imagepatterns. In particular, intra mode dependent non-separable primaryand/or secondary transform schemes are disclosed. These exampleembodiments focus on data scanning orders in blocks being coded/decodedeither during the forward or inverse transform processes. A block beingprocessed may be generally referred to as a data block, which, forforward transforms, may include samples of a residualtransform/coding/prediction block or primary transform coefficients, andfor reverse transforms, may include secondary transform coefficients tobe inverse transformed into primary transform coefficients or primarytransform coefficients to be inverse transformed to residual samples.

In order to optimize the energy compaction achieved by the non-separableprimary or secondary transform and to effectively quantize the primaryor secondary transform coefficients (to discard high frequencycoefficients), the input and output scanning processes of thenon-separable primary or secondary transform may be performed with intraprediction mode, block size and/or primary transform type taken intoconsideration, as described in further detail below.

In some example implementations, a transform set may refer to a group ofone or more transform kernels as candidates or options for an encoder toselect for a data block during coding process.

In some implementations, the primary transform may be performed usingnon-separable transform, or achieved by performing a series of 1-Dtransforms. For example, in a DCT_DCT combination, a DCT is appliedhorizontally and vertically on a block. For another example, in anADST_ADST combination, a 1-D ADST is applied horizontally and verticallyon a block. In some implementations, different transform types may beused horizontally and vertically. Such transforms may be referred ashybrid primary transforms.

FIG. 15 shows example data flow 1500 for forward and inversenon-separable transform. The various data scanning processes areidentified as S1 and S2 and the forward and inverse non-separabletransformations are identified b 1502 and 1504 in FIG. 15 . Theoperation of the example data flow 1500 applies to both primary andsecondary non-separable transform processes and are explained in furtherdetail below.

Data Scanning in Non-Separable Secondary Transform

In this example embodiment, the forward and inverse secondary transformsmay be non-separable transforms, as shown by 1502 and 1504 of FIG. 15 .From the encoding side, the input of the forward secondary transform1502 may be N primary transform coefficients 1506 of a primarycoefficient data block 1508 scanned into data sequence 1510 following afirst scanning order S1 in the scanning process 1512, and the output ofthe forward secondary transform 1502 may be secondary transformcoefficient sequence 1514 of K data items which replace K input primarytransform coefficients 1516 of the primary transform coefficients togenerate a modified primary transform coefficient data block 1507following a data scan 1518 using a second scanning order S2. N and K arepositive integers.

From the decoding side, the input of the inverse secondary transform1504 is the secondary transform coefficient sequence 1520 of K dataitems obtained following reversely scanning the K input coefficients1517 of the modified primary coefficient data block 1509 in the inverseS2 scanning order in a scanning process 1522, and the output of theinverse secondary transform 1504 may be primary transform coefficientsequence 1524 of N coefficients which replace N primary transformcoefficients 1526 of 1509 to generate primary coefficient data block1528 using inverse scanning process 1530 following the inverse S1scanning order.

In some example implementations, N may be larger than K. In other words,the non-separable secondary transform 1502 may be a reducednon-separable secondary transform. In some example embodiment, the Nprimary transform coefficients 1506 may be the first N coefficients ofthe primary coefficient data block 1506, e.g., the upper left corner (orlow frequency portion) of the primary transform coefficient data block1506.

In one example implementation, the scanning order S1 may depend on atleast one of: an intra prediction mode associated with the input datablock 1508, a primary transform type associated with the data block, ora block size of the data block.

In one example implementation, the scanning order S2 may depend on atleast one of: an intra prediction mode associated with the input datablock 1508, a primary transform type associated with the data block, ora block size of the data block.

In one example implementation, the S1 and S2 scanning order may includeat least one of: a zig-zag scanning order, a diagonal scanning order, ora row and column scanning order.

In one implementation, when a same secondary transform set (having oneor more second transform kernels) is used as secondary transformcandidates for multiple intra prediction modes, the S1 and S2 scanningorder may depend on the intra prediction mode associated with the datablock.

In some example implementations, the S1 and S2 scanning orders may bedifferent.

In some example implementations, N and K include but not limited to anyinteger between 0 and 127, inclusive.

In some example implementations, when the data block is a square block,S1 and/or S2 scanning order may include a zig-zag order. When the blockis a non-square block, S1 and/or S2 scanning order may include adiagonal scan order.

FIG. 15 further shows the quantization and entropy encoding 1540 and thecorresponding entropy decoding and dequantization 1550.

Data Scanning in Non-Separable Primary Transform as Primary Transform

In this example embodiment, the non-separable transform process 1502 and1504 may be performed with respect to primary transform. As such, theinput to the data flow of 1500 (1508) would be, e.g., residual sampledata block rather than primary transform coefficient and the output 1507would be modified residual sample data block that contains K primarytransform coefficients 1516 from the non-separable transform process1502. Specifically, from the encoder side, the input of the forwardprimary transform may be the first N residual samples following a firstscanning order S1, and the output of the forward primary transform are Knon-separable transform coefficients which replace K input residualsamples in a second scanning order S2. N and K are positive integers.From the decoder side, the input of the inverse non-separable transformis K non-separable transform coefficients following the S2 scanningorder, and the output of the inverse non-separable transform is Nresidual samples which replace N non-separable transform coefficientsfollowing the S1 scanning order, where N is an integer.

In one example implementation, the scanning order S1 may depend on atleast one of: an intra prediction mode associated with the data block, aprimary transform type associated with the data block, or a block sizeof the data block.

In one example implementation, the scanning order S2 may depend on atleast one of: an intra prediction mode associated with the data block, aprimary transform type associated with the data block, or a block sizeof the data block.

In one example implementation, the S1 and S2 scanning order may includeat least one of: a zig-zag scanning order, a diagonal scanning order, ora row and column scanning order.

In one example implementation, when a same transform set (having one ormore second transform kernels) is used as secondary transform candidatesfor multiple intra prediction modes, the S1 and S2 scanning order maydepend on the intra prediction mode associated with the data block.

In some example implementations, the S1 and S2 scanning orders may bedifferent.

In one example implementation, N and K include but not limited to anyinteger between 0 and 127, inclusive.

In one example implementation, when the data block is a square block, S1and/or S2 scanning order may include a zig-zag scanning order. When theblock is a non-square block, S1 and/or S2 scanning order may include adiagonal scanning order. In other words, a diagonal scan order may beforced for either square or non-square data blocks.

In one example implementation, the S1 scanning order may include: ahorizontal (row) scanning order, or a vertical (column) scanning order.

For these implementations, the quantization/entropy process 1540 andentropy decoding/dequantization process 1550 may be directly performedon 1507 or 1509 without additional secondary transform. In some otheralternative implementations, additional secondary transform may beperformed preceding the process 1540 on the encoding side and followingthe process 1550 on the decoding side. The additional secondarytransform may be separable, or may be non-separable, e.g. following theimplementations described above for non-separable secondary transform.

Data Scanning in Non-Separable Primary Transform as Primary Transform

In some embodiments, the type of transform (i.e., separable ornon-transferable) may have an impact on the scanning order when performthe transform coefficient coding (e.g., entropy encoding of thetransform coefficient). In one implementation, when non-separabletransform is applied (to the primary transform and/or the secondarytransform), the horizontal (row) or vertical (column) scanning order maybe used for transform coefficient coding.

FIG. 16 shows an exemplary method 1600 for processing video data. Themethod 1600 may include a portion or all of the following step: step1610, extracting a data block from the video data; step 1620, scanning afirst number of data items in the data block following a first scanorder to generate a first data sequence; step 1630, performing anon-separable transform to the first data sequence to obtain a seconddata sequence having a second number of data items; and step 1640,replacing at least a portion of the first number of data items in thedata block with a portion or all of the second data sequence following asecond scan order.

In embodiments of this disclosure, any steps and/or operations may becombined or arranged in any amount or order, as desired. Two or more ofthe steps and/or operations may be performed in parallel.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium. Embodiments in the disclosure may be appliedto a luma block or a chroma block.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1722),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include an interface (1754) to one ormore communication networks (1755). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CAN bus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general-purpose data ports or peripheral buses (1749) (such as,for example USB ports of the computer system (1700)); others arecommonly integrated into the core of the computer system (1700) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (1700) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), graphicsadapters (1750), and so forth. These devices, along with Read-onlymemory (ROM) (1745), Random-access memory (1746), internal mass storagesuch as internal non-user accessible hard drives, SSDs, and the like(1747), may be connected through a system bus (1748). In some computersystems, the system bus (1748) can be accessible in the form of one ormore physical plugs to enable extensions by additional CPUs, GPU, andthe like. The peripheral devices can be attached either directly to thecore's system bus (1748), or through a peripheral bus (1749). In anexample, the screen (1710) can be connected to the graphics adapter(1750). Architectures for a peripheral bus include PCI, USB, and thelike.

CPUs (1741), GPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As a non-limiting example, the computer system having architecture(1700), and specifically the core (1740) can provide functionality as aresult of processor(s) (including CPUs, GPUs, FPGA, accelerators, andthe like) executing software embodied in one or more tangible,computer-readable media. Such computer-readable media can be mediaassociated with user-accessible mass storage as introduced above, aswell as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1740). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1740) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1746) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1744)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

APPENDIX A: ACRONYMS

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

HDR: high dynamic rangeSDR: standard dynamic range

JVET: Joint Video Exploration Team

MPM: most probable mode

WAIP: Wide-Angle Intra Prediction CU: Coding Unit PU: Prediction UnitTU: Transform Unit CTU: Coding Tree Unit PDPC: Position DependentPrediction Combination ISP: Intra Sub-Partitions SPS: Sequence ParameterSetting PPS: Picture Parameter Set APS: Adaptation Parameter Set VPS:Video Parameter Set DPS: Decoding Parameter Set ALF: Adaptive LoopFilter SAO: Sample Adaptive Offset CC-ALF: Cross-Component Adaptive LoopFilter CDEF: Constrained Directional Enhancement Filter CCSO:Cross-Component Sample Offset LSO: Local Sample Offset LR: LoopRestoration Filter AV1: AOMedia Video 1 AV2: AOMedia Video 2

What is claimed is:
 1. A method for processing video data, the methodcomprising: extracting a data block from the video data; scanning afirst number of data items in the data block following a first scanorder to generate a first data sequence; performing a non-separabletransform to the first data sequence to obtain a second data sequencehaving a second number of data items; and replacing at least a portionof the first number of data items in the data block with a portion orall of the second data sequence following a second scan order.
 2. Themethod of claim 1, wherein: the data block comprises primary transformcoefficient; the non-separable transform comprises a forwardnon-separable secondary transform; and replacing at least a portion ofthe first number of data items in the data block comprises replacing thesecond number of data items within the first number of data items in thedata block with the second sequence following the second scan order. 3.The method of claim 2, wherein the first scan order is determined basedon at least one of: an intra prediction mode associated with the datablock; a type of the forward non-separable secondary transform; or asize of the data block.
 4. The method of claim 2, wherein the secondscan order is determined based on at least one of: an intra predictionmode associated with the data block; a type of the forward non-separablesecondary transform; or a size of the data block.
 5. The method of claim2, wherein the first scan order or the second scan order comprises oneof: a zig-zag scan order; a diagonal scan order; or a row and columnscan order.
 6. The method of claim 2, wherein: the first scan order orsecond scan order is determined based on an intra prediction modeassociated with the data block when a same set of transform kernels areshared by multiple intra prediction modes with respect to the videodata.
 7. The method of claim 2, wherein the first number and the secondnumber are integers between 0 to 127, inclusive.
 8. The method of claim2, wherein the first scan order and the second scan order are: a zig-zagscan order when the data block is a square block; or a diagonal scanorder when the data block is a non-square block.
 9. The method of claim1, wherein: the data block comprises residual samples; the non-separabletransform comprises a forward non-separable primary transform; andreplacing at least a portion of the first number of data items in thedata block comprises replacing the second number of data items withinthe first number of data items in the data block with the secondsequence following the second scan order.
 10. The method of claim 9,wherein the first scan order is determined based on at least one of: anintra prediction mode associated with the data block; a type of theforward non-separable secondary transform; or a size of the data block.11. The method of claim 9, wherein the second scan order is determinedbased on at least one of: an intra prediction mode associated with thedata block; a type of the forward non-separable secondary transform; ora size of the data block.
 12. The method of claim 9, wherein the firstscan order or the second scan order comprises one of: a zig-zag scanorder; a diagonal scan order; or a row and column scan order.
 13. Themethod of claim 9, wherein: the first scan order or second scan order isdetermined based on an intra prediction mode associated with the datablock when a same set of transform kernels are shared by multiple intraprediction modes with respect to the video data.
 14. The method of claim9, wherein the first number and the second number are integers between 0to 127, inclusive.
 15. The method of claim 9, wherein the first scanorder and the second scan order are: a zig-zag scan order when the datablock is a square block; or a diagonal scan order when the data block isa non-square block.
 16. The method of claim 9, wherein the first scanorder is a horizontal or vertical scan order.
 17. A method for entropyencoding transform coefficients associated with video data, the methodcomprising: in response to a transform associated with the transformcoefficients being non-separable, scanning the transform coefficientwhen performing entropy encoding of the transform coefficients using afirst scanning order being one of: a horizontal scan order; or avertical scan order; and in response to the transform associated withthe transform coefficients being separable, scanning the transformcoefficient in a second scanning order different from the first scanningorder when performing entropy encoding of the transform coefficients.18. The method of claim 17, wherein the transform comprises one of: aprimary transform; or a secondary transform.
 19. A device comprising acircuitry configured to implement the method of claim
 1. 20. A methodfor processing video data, the method comprising: receiving the videodata; determining whether a non-separable transform is applied as asecondary transform to the video data; in response to the non-separabletransform being applied as the secondary transform to the video data:scanning a first number of primary transform coefficients, wherein theprimary transform coefficients follow a first scanning order; performinga non-separable transform using the first number of primary transformcoefficients as an input to obtain a second number of secondarytransform coefficients as an output, wherein the secondary transformcoefficients follow a second scanning order; replacing at least thesecond number of primary transform coefficients with the secondarytransform coefficients following the second scanning order; performingan inverse secondary transform corresponding to the non-separabletransform using the second number of secondary transform coefficients asan input to obtain the first number of primary transform coefficients asan output; and replacing at least the first number of secondarytransform coefficients with the primary transform coefficients followingthe first scanning order.